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First the two boys weighing 50lbs cross the river together. Once across, one boy gets out while the other rows back across to the boy who weighs 100lbs. The 50lb boy gets out and the 100lb boy gets in and rows across the river alone. Once the 100 lb boy makes it across, he gets out of the boat and the 50lb boy gets in, rows back across to the other 50 lb boy. Now both 50 lb boys may cross together again.